A bivalve shell has the two valves which constitute it connected at the ba sis, or inferior margin : opposite to which is the superior margin : the shell being supposed to be placed on the hinge, its margin is divided into the anterior and the posterior margin. The length of the shell is measured from the inferior to the superior margin, and the breadth from the anterior to the posterior margin. The belly is the most tumid part, and the beaks, the prominences over the hinge, are of a conoidal and spiral form. At the basis of the shell is the hinge, by which the valves are moved on each other; and in the anterior chink &ligament is placed, by which the valves are closely connect ed. This chink is formed by a slight se paration of the valves at this, the anteri or part, which having a distinct surface from the general disk, and being indeed separated from it by an angle, or by a rais ed or sunk line, it may he distinguished as the corslet ; this surface is particularly distinguished in the shells of the Venus kind. On these parts, in some species, are spines, in others various markings, and in others these parts are pl.,in. At the lower part of the anterior margin of the valves, within the limits of the curs. let, and above the ligament, is a part ge nerally distinguishable from the rest of the surface by a difference in the colour, striae, &c. This part varies in its form, being canaliculated, replicated, &c. Its edges are termed On the other side of the hinge, on the posterior part, and near the hinge, is ge nerally a lunated depression ; the cres cent, like the part just described, va• ries in its form and markings in different shells. Both these parts derive half their form from each of the valves ; and are consequently separated in the middle.
To determine the side to which a valve belongs, the shell need only be placed on the hinge, with the anterior side forward, when it directly shows itself.
In sonic irregular shells, as of oysters, and spondyles, the shells are divided into upper and under, the upper shell being flatter than the under. In the Terebra tulz, the tippet shell, the beak of which is pierced, is more tumid than the under: but in most shells, as in the Pectens, and the oysters, the 'upper valve is almost al ways the least tumid.
The hinge is, in general, furnished with teeth ; but sometimes it is without. When placed on the side, they are said to be lateral ; and when on the inferior extremity, terminal. The cen. tral teeth, essential to the genus, are termed the cardinal teeth ; and the others, accessory or secondary. The teeth are in some shells articulated, in a cavity, in the opposite valve ; in others, they are not. In multivalves also the shell is the combination of all the valves, whether connected by articu lation, or by ligaments.
From the observations and experiments of Iteatimur, which have been since con siderably extended by Bruguiere and others, the formation and growth of shells have obtained considerable illustration. Leuwenhoeck, Swammerdam, Lister, and others, had observed, that, at the first escape of the animal from the egg, it was invested by a complete spiral torn, at least of the shell: and it appears, from the ob servations of Adanson, that viviparous shell-fish are likewise bmught forth in a similar state. The growth of shells was discovered by Reaumur to take place in that which is most distant from the first formed part. Here a part of the animal
exists, which is not yet covered with shell, but which is beset with a vast number of vessels, which separate, and deposit on the existing edge of the shell those glu tinous and calcareous matters, by which its due increased extent of surface is ob tained. In proof of this being the process by which the augmentation of the shell is effected, he broke the shells of various living testaceous animals in different parts, and was thus enabled to perceive, that the newly added matter was not deposit. eel line after line from the shell, at the edges of the fractured part ; but was se parated, in a pellicle, from the body the animal, and thus applied at once to the whole of the vacuity. Those, who de nied this mode of increase, denied also the removal of the posterior termination of the worm, from the extreme termination of the first formed spire. But it is cer tain, that the animals in many shells have their posterior termination attached to the point of the shell only in their earliest stages ; and that, when older, it is found adherent to the second or third turn : and it has also been ascertained that the tail of the Nerite is attached beneath the left or columellar lip. In proof of these ani mals possessing this power of removing themselves from these turns of the shell, or indeed from those parts in which their residence is no longer necessary, the cir cumstance may be adduced of the animal belonging to the Porcellanea, sometimes abandoning its old shell, and forming a new one.
The various tints of different colours with which shells are so beautifully adorned, result, according to the remarks of the same ingenious naturalist, from an economy and organization somewhat si milar to that which has been just men tioned. On the neck of the animal, that part from which the matter of the shell is supposed to be secreted,•the colours of the shell may be detected. Thus, if the ground of the shell be yellow, and it be marked with dark brown or black bands, then the neck of the animal will be seen of a white, inclining to a yellow hue, wit Ii dark spots, answering in their number and direction to the bands observable in the shell itself: the colouring matter ap pearing to be here disposed, ready for its deposition, with the other substances of the shell. In this manner, and on these principles, has Reaumur been able to ex plain almost every circumstance, respect. mg the varieties of form and of colour which are observable in these But a circumstance is observable in the Olives and the Porcellanea., to which Reaumur has not attempted to apply his doctrine. In these shells the colours are disposed in two layers, the outer of which is the production of an organization dif. ferent froth that of any of the other inha bitants of shells, and from which pro ceeds an operation, also different from what occurs in any other instance. At the period at which Reaumur wrote, se veral of the olives and of the cowries, in consequence of certain differences of co lour, and of some difference also in their forms, were considered as of different species from others, of which they were in fact individuals of the same species, but had not yet acquired their perfect state. For want of reckoning upon this circumstance, the celebrated Linnzus has unnecessarily multiplied the number of species of these shells.